1. Field of the Invention
This invention relates to a method for discharging a polymer slurry. More specifically, it pertains to a method of smoothly and substantially continuously discharging a polymer slurry formed in a continuous polymerization process from a polymerization vessel or a washing vessel and the like used in a post-treatment step.
2. Description of the Prior Art
According to conventional techniques, so called low or medium pressure process, olefin polymers such as ethylene polymers or propylene polymers are generally obtained by the continuous slurry polymerization of olefins such as ethylene or propylene in a solvent at a temperature below the melting point of the resulting polymer, i.e. below 100.degree. C., in the presence of a Ziegler-Natta catalyst system or a metal oxide-type catalyst system. The polymer obtained by the polymerization precipitates in the solvent to form a polymer slurry. Paraffinic or olefinic hydrocarbons having at least 3 carbon atoms are generally used as the solvent. The polymer formed commercially generally has an average particle diameter of 0.1 to 1 mm and a bulk density of 0.3 to 0.53 kg/liter although its properties differ depending upon the catalyst and polymerization temperature employed. The shape and particle size distribution of the polymer usually differ greatly depending upon the catalyst used. The concentration of the polymer in the polymer slurry is usually 200 to 1,000 g/liter of solvent. Needless to say, the long-term smooth operation of a polymer manufacturing plant depends greatly upon the ability to prevent clogging of valves and piping systems in handling the polymer slurry.
In discharging a polymer slurry, it has been the previous practice to use a ball valve, a butterfly valve, a flash valve directly inserted into the main body of the vessel as shown in FIG. 1, or an angle valve as shown in FIG. 2 adapted to prevent clogging by the polymer as a discharge valve, and repeating the complete opening and complete closing of the valve intermittently over a short period of time. However, since according to the method involving operating the valve intermittently, a large quantity of polymer slurry is drawn off at the time of discharge, the condition of the inside of the polymerization vessel or washing vessel located upstream is disturbed, and the condition of the inside of the polymer slurry receiving tank on the downstream side also fluctuates greatly and a load is exerted on it. To absorb these fluctuations, the capacities of the receiving tank and the like on the downstream side must be increased. This inevitably leads to an increase in the cost of equipment and in installation space.
In an attempt to overcome these disadvantages, suggestions were made to operate the valves stably and continuously. Some were successful in short-term operations, but it was difficult to perform long-term operations without any trouble. In particular, when the flow rate is small, the degree of valve opening must be decreased to a small size and as a result it is difficult for the polymer particles to pass through the valve and clogging occurs within a short period of time.